The long-term goal of this project is to determine the structural changes that underlie K+ channel function. These membrane-spanning proteins are critical in controlling the electrical potential difference across the membrane, which, in turn, forms the basis for solute exchange and cellular excitability. It follows that a full understanding of the relation between structure and function in K+ channels is a high priority. Achieving this goal will require knowledge of the conformational changes that mediate two key properties, ion permeation and gating. In this proposal, we will characterize at the atomic level these two properties by trapping the channel complex in different kinetic states. Subsequent structural analysis will address the following fundamental but unanswered questions: 1) what is the high-resolution structure of KcsA trapped in the open and C-type inactivated state? 2) what is the amino-acid network underlying allosteric communication between the activation gate and the selectivity filter of KcsA? and 3) which are the structural changes of the KcsA selectivity filter that lead to C-type inactivation? The answers to these questions will provide us with the structural background needed to understand ion selectivity, permeation, and gating of K+ channels in a dynamic context and eventually will assist in the rational design of drugs for the treatment of K+ channel- related diseases. We will focus our study on an archetypal prokaryotic channel, KcsA, that contains all the structural elements characteristic of K+ channel function (i.e., ion permeation, activation, and C-type inactivation gating) but possesses a structure simple enough to facilitate analysis. A dual approach combining crystallographic and electrophysiological methods will provide high-resolution functional and structural information for this model channel in various kinetic states.